1. Field of the Invention
The present invention relates to a method of epitaxial-like bonding of wafer pairs at low temperature, and more particularly to a method of bonding in which the wafer surfaces are modified to create surface and subsurface defect areas, and possibly amorphized, by ion implantation or plasma, preferably by boron-containing ions or a plasma such as B2H6.
2. Discussion of the Background
For many optoelectronic and electronic device applications, homo-epitaxial single crystalline layers consisting of same material with same crystalline orientation but different doping types or levels are necessary. For some device applications, active layers comprising single crystalline dissimilar materials are required. The active layers should be high crystallographic quality with interfaces that are thermally conductive and almost optical loss free. Conventional hetero-epitaxial growth techniques applied to these lattice mismatched active layers usually result in a large density of threading dislocations in the bulk of the layers. Bonding of single crystalline wafers of identical or dissimilar materials is an unique alternative approach to the epitaxial growth. Not only highly lattice-mismatched wafers can be bonded but also wafers with different crystalline orientations can be combined. Ideally, the mismatches of single crystalline bonding wafers are accommodated by dislocations (in lattice-mismatch case) or an amorphous layer (in orientation-mismatch case) localized at the bonding interface with no defects generated in the bulk area. This approach is termed epitaxial-like bonding. The epitaxial-like bonding can also be employed to prepare unique devices by integrating already processed device layers.
However, conventional epitaxial-like bonding is achieved by high temperature annealing. To bond wafers composed of thermally mismatched materials, severe and often damaging thermal stresses can be induced with high temperature annealing. Since thermal stresses can increase significantly with the size of dissimilar wafers, only small wafers currently can be epitaxially bonded at high temperatures. The high temperature annealing process can also produce unwanted changes to bonding materials and often prevents the bonding of processed device wafers. The bonding materials may decompose at high temperatures, even if the bonding wafers are thermally matched.
In order to epitaxially bond large wafers of dissimilar materials or processed wafers, an epitaxial-like bonding interface must be achieved at or near room temperature, or one wafer of the bonded pair must be thinned sufficiently before annealing to elevated temperatures. Although Gxc3x6esele et al. in Applied Physics Letters 67, 3614 (1995) and Takagi et al in Applied Physics Letters 74, 2387, 1999 reported room temperature epitaxial-like bonding of silicon wafers in ultrahigh vacuum, high temperature ( greater than 600xc2x0 C.) pre-annealing in the former case or high external pressure ( greater than 1 MPa) in the latter case were required to achieve the bond that may introduce undesired effects to the bonding wafers.
Recently, M. Bruel in Electronics Letters 31,1201 (1995) reported a promising generic thinning approach using a hydrogen-induced layer transfer method (so-called smart-cut method). In this approach, H atoms are implanted into a Si wafer to such concentration that a significant fraction of Sixe2x80x94Si bonds are broken creating a buried H-rich layer of micro-cracks susceptible to cleavage or fracture. By bonding the topmost oxide covered hydrophilic Si wafer surface to another substrate, a thin layer of the Si wafer can then be transferred by fracture of the H-rich region. However, this process requires that the bonding energy between the bonded wafers be higher than the fracture energy of the hydrogen-induced crack region at the layer transfer temperature. The layer transfer temperatures must be lower than the temperature beyond which hydrogen molecules in the material become mobile. For silicon, the temperature is about 500xc2x0 C. (see Chu et al in Physics Review B, 16, 3851 (1987)). The bonding energy of conventional HF dipped hydrophobic silicon wafer pairs is higher than the hydrogen-induced region only after annealing at temperatures higher than 600xc2x0 C. Therefore, this process does not work for oxide-free hydrophobic silicon wafer bonding.
Typically, HF-dipped, hydrogen-terminated hydrophobic silicon wafers are used to realize epitaxial-like bonding after annealing at  greater than 700xc2x0 C. In order for bonded hydrophobic silicon wafer pairs to reach bulk fracture energy, Tong et al. in Applied Physics Letters 64, 625 (1994) reported that hydrogen (from HF-dip, mainly Sixe2x80x94H2 and Sixe2x80x94H terminated hydrophobic silicon surfaces at the bonding interface) must be removed so that strong Sixe2x80x94Si epitaxial bonds across the mating surfaces can be formed. The reaction is illustrated in Equation (1).
Sixe2x80x94H+Hxe2x80x94Sixe2x86x92Sixe2x80x94Si+H2xe2x80x83xe2x80x83(1)
The release of hydrogen from a stand-alone silicon wafer dipped in HF was demonstrated to start at about 367xc2x0 C. from Sixe2x80x94H2 and 447xc2x0 C. from Sixe2x80x94H in an ultrahigh vacuum. Since hydrogen molecules become mobile in silicon only at temperatures higher than 500xc2x0 C., annealing at temperatures higher than 700xc2x0 C. have been found necessary to completely deplete hydrogen from the bonding interface that results in a high bonding energy. Therefore, the smart-cut method for a layer transfer using conventional HF-dipped silicon wafer pairs is not possible because the bonding energy is too low at layer transfer temperatures that are lower than 500xc2x0 C.
Based on above arguments, it becomes clear that the development of a low temperature epitaxial-like wafer bonding technology that is both cost-effective and manufacturable is essential for many advanced materials and device applications.
Accordingly, one object of this invention is to provide a wafer bonding method and bonded structure in which epitaxial-like bonding is achieved at near room temperature in ambient conditions without an external pressure.
Another object of this invention is to provide a wafer bonding method and bonded structure using bonding surfaces treated to obtain amorphized or partially amorphized surfaces by ion implantation or plasma, preferably by boron-containing ions or plasma.
These and other objects of the present invention are provided by a method for bonding first and second substrates including steps of preparing substantially oxide-free first and second surfaces of respective first and second substrates, creating a surface defect region in each of said first and second surfaces, and bonding said first and second surfaces. Creating the defective region may include plasma-treating the first and second surfaces of the first and second substrates with a plasma, and preferably a boron-containing plasma. The plasma-treating step may utilize a plasma in reactive ion etch (RIE) mode using B2H6 gas, and possibly a mixture of B2H6, He, and Ar gases. Other gas plasmas, such as Ar may also be used.
As a result of the plasma-treating step, a thin amorphous layer may be formed in the first and second surfaces. A monolayer of boron may also be on said first and second surfaces and first and second surfaces may be doped with boron when a boron-containing plasma is used. Also, a few monolayers of boron are introduced into each of the first and second surfaces during said plasma-treating step.
After contacting, the substrates are maintained in contact, preferable under low vacuum but also in ambient air. A bonding energy of about 400 mJ/m2 may be obtained at room temperature. Also, when the bonded pair of substrates is maintained at a temperature no more than about 250xc2x0 C. after contacting, a bond strength of at least about 1500 mJ/m2 may be obtained, and a bonding energy of about 2500 mJ/m2 (bulk silicon fracture energy) may be obtained at 350xc2x0 C. The method may also include step of annealing said bonded first and second surfaces at a temperature in the range of about 250-450xc2x0 C., or at a temperature not exceeding about 350xc2x0 C. A substantial portion of said amorphous layers in said first and second surfaces may be recrystallized, possibly in a separate annealing step.
The creating step may also include ion-implanting the first and second surfaces. As or B may be used to implant the surfaces. In the case of B, a surface layer is formed on the surface of the substrate and the energy of the implant is chosen to place the peak of the concentration profile at approximately the interface between the substrate surface and the surface layer. In the case of As, the surface is directly implanted and a thicker amorphous layer may be formed.
After contacting, the implanted substrates are maintained in contact, preferable under low vacuum but also in ambient air. The bonded pair may be heated at a temperature no more than about 400xc2x0 C. The bonded pair of substrates may be maintained at a temperature no more than about 400xc2x0 C. after contacting. A bonding energy of about 2500 mJ/m2 (bulk silicon fracture energy) may be obtained.
To obtain the substantially oxide-free surfaces, the substrates may be immersed in a first etching solution, such as a hydrofluoric acid solution, before said plasma-treating step, and immersed in a second etching solution, such as a hydrofluoric acid solution, after said plasma-treating step. The substrates may be cleaned before immersing in the first etching solution, preferably using an RCA-1 solution.
The method may also include plasma treating an exposed surface of the bonded pair of substrates in a boron-containing plasma, and bonding a third wafer to said exposed surface.
The method may also include creating the defect region or amorphous layer in the surface of a silicon layer formed on a semiconductor device wafer. Two or more of the treated wafers may be bonded together.
The first and second substrates may be selected from Si, InGaAs, Inp; GaAs, Ge, SiC and other semiconductors.
The objects of the invention may also be achieved by a method including amorphization of first and second surfaces of first and second silicon substrates by ion implantation or plasma, and contacting the first and second surfaces to form a bonded pair of substrates. The substrates may be immersed in a first hydrofluoric acid solution before said amorphization step, and immersed in a second hydrofluoric acid solution after said amorphization step by plasma. The substrates may be cleaned before immersing in the first hydrofluoric acid solution, preferably using an RCA-1 solution. The amorphization step may utilize arsenic (As) ion implantation or argon (Ar) RIE plasma.
After contacting, the substrates are maintained in contact, preferable under low vacuum but also in ambient air. The bonded pair may be heated at a temperature no more than about 400xc2x0 C. The bonded pair of substrates may be maintained at a temperature no more than about 400xc2x0 C. after contacting. A bonding energy of about 2500 mJ/m2 (bulk silicon fracture energy) may be obtained. The bonded amorphous layers at the bonding interface can be completely recrystallized after annealing at 450xc2x0 C.
The objects of the invention may also be achieved by a bonded structure having a first substrate having a first surface, a first amorphous layer formed in the first surface, and a second substrate having a second surface, a second amorphous layer formed in the second surface. The first surface is bonded to the second surface to form a bonded pair of substrates.
One of said first and second substrates of said bonded pair may have a third surface with an amorphous layer. A third substrate having a fourth surface, with a fourth amorphous layer formed in said fourth surface may be bonded to the bonded pair.
The first and second surfaces may comprise a surface exposed to boron-containing plasma. One of the first and second substrates of the bonded pair may have a planar surface exposed to boron-containing plasma bonded to a third surface of a third substrate exposed to boron-containing plasma. The first, second and third substrates may be selected from Si, InGaAs, InP, GaAs, Ge, SiC and other semiconductors.
The first and second substrates may comprise respective first and second semiconductor devices, the first surface may comprise a substantially planar surface of a first silicon layer formed on the first device, and the second surface may comprise a substantially planar surface of a second silicon layer formed on the second device.
The first surface may comprise a first silicon surface exposed to an inert gas plasma, and the second surface may comprise a second silicon surface exposed to an inert gas plasma.
The first surface may comprise a first silicon surface implanted with boron, and the second surface may comprise a second silicon surface implanted with boron.
The first surface may comprise a first silicon surface implanted with arsenic, and the second surface may comprise a second silicon surface implanted with arsenic.
The bonded structure according to the invention may include a first substrate having an amorphized first surface and containing boron and a second substrate having an amorphized second surface and containing boron, with the first surface being bonded to the second surface to form a bonded pair of substrates. The first and second substrates may be selected from Si, InGaAs, InP, GaAs, Ge, SiC and other semiconductors.
The first surface may comprise a first silicon surface exposed to a boron-containing plasma, and the second surface may comprise a second silicon surface exposed to a boron-containing plasma.
The first surface may comprises a first silicon surface implanted with boron, and the second surface may comprise a second silicon surface implanted with boron.
The bonded structure may also have a first substrate having a first surface implanted with boron, and a second substrate having a second surface implanted with boron. The first surface is bonded to said second surface to form a bonded pair of substrates. One of the first and second substrates of the bonded pair may have a planar surface implanted with boron. A third surface, implanted with boron, of a third substrate may be bonded to the planar surface. The first, second and third substrates may be selected from Si, InGaAs, InP, GaAs, Ge, SiC and other semiconductors.